This invention relates generally to composite materials, and more specifically to composite panels having a honeycomb core that are manufactured using a single-step curing process.
Honeycomb core sandwich panels include composite laminate skins co-cured with adhesives to a honeycomb core are known. Such panels are utilized, for example, in the aerospace industry. Such honeycomb core sandwich panels are used at least partially due to their high stiffness-to-weight (i.e., specific stiffness) and strength-to-weight (i.e., specific strength) ratios. Current sandwich panels are processed, for example, using multiple cure cycles, or are co-cured in one step at low cure pressures in order to prevent deformation of the honeycomb core.
When multiple cure cycles are used, either one or both of the composite skins are precured, then later bonded or cobonded onto the honeycomb core and/or the other skin. The multiple and complex steps increase production expenses and also increase the risk of defective panels due to, for example, bondline discrepancies, since the precured skin must match very closely with the contour-machined honeycomb core surface.
A co-cure process refers to more than one skin simultaneously cured and bonded onto the machined honeycomb core pieces. A skin may include one ply of a composite material or a plurality of plies of a composite material. Numerous quality issues typically occur with a co-cure process such as, for example, distortion of the composite plies and/or dispersion of the composite resin into cells of the honeycomb core, as well as distortion of cells of the honeycomb core from their pre-processed core cell shape. Other quality issues due to known co-cure processes may also include formation of porous composite skins caused by curing at a less than optimum pressure, and also due to distortion of the composite skins into the cells of the core.
Furthermore, quality inspections of composite panels formed using known co-cure processes are complicated because it may be difficult to accurately assess the quality of a composite panel formed using a known co-cure process using typical non-destructive inspection techniques, such as ultrasonic inspection methods.
There are no current composite/honeycomb sandwich design and process schemes in which a one-step co-cure process produces a high strength structural part, with substantially porosity-free composite skins, minimal resin leakage into the honeycomb core, minimal honeycomb core distortion, minimal composite ply distortion into the honeycomb cells, and having composite skins that may be readily inspected by known ultrasonic inspection techniques to verify the co-cured skins are substantially porosity-free.